Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments			4
Net financing revenue and other interest income	$ 1,157	$ 1,094	$ 2,289	$ 2,143
Other revenue	395	364	861	718
Total net revenue	1,552	1,458	3,150	2,861
Provision for loan losses	177	158	459	419
Noninterest expense	881	839	1,711	1,653
Income from continuing operations before income tax expense	494	461	980	789
Assets	180,448	171,345	180,448	171,345	$ 178,869
Net financing revenue and other interest income after provision for loan losses	980	936	1,800	1,700
Operating Segments | Automotive Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income	1,022	925	2,002	1,834
Other revenue	61	63	129	129
Total net revenue	1,083	988	2,131	1,963
Provision for loan losses	180	170	442	429
Noninterest expense	444	436	901	884
Income from continuing operations before income tax expense	459	382	788	650
Assets	114,955	114,915	114,955	114,915
Operating Segments | Insurance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income	15	13	27	25
Other revenue	286	266	646	512
Total net revenue	301	279	673	537
Provision for loan losses	0	0	0	0
Noninterest expense	301	268	528	499
Income from continuing operations before income tax expense	0	11	145	38
Assets	8,241	7,634	8,241	7,634
Operating Segments | Mortgage Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income	46	44	96	87
Other revenue	4	2	6	3
Total net revenue	50	46	102	90
Provision for loan losses	0	0	2	2
Noninterest expense	36	32	73	66
Income from continuing operations before income tax expense	14	14	27	22
Assets	16,584	13,385	16,584	13,385
Operating Segments | Corporate Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income	61	57	115	103
Other revenue	10	14	21	22
Total net revenue	71	71	136	125
Provision for loan losses	3	(6)	26	(6)
Noninterest expense	22	19	51	44
Income from continuing operations before income tax expense	46	58	59	87
Assets	4,980	4,458	4,980	4,458
Corporate and Other
Segment Reporting Information [Line Items]
Net financing revenue and other interest income	13	55	49	94
Other revenue	34	19	59	52
Total net revenue	47	74	108	146
Provision for loan losses	(6)	(6)	(11)	(6)
Noninterest expense	78	84	158	160
Income from continuing operations before income tax expense	(25)	(4)	(39)	(8)
Assets	$ 35,688	$ 30,953	$ 35,688	$ 30,953